A fuel-oil heat exchange (FOHE) system is often provided in aircraft gas turbine engines to exchange heat between the cold fuel and hot oil, in order to cool the hot oil and heat the cold fuel prior to combustion, which enhances engine efficiency. In prior art FOHE systems, a single FOHE is usually provided in a fixed size. A main hot oil flow goes through the sole FOHE at all times, regardless of fuel temperatures, when the fuel flow is discharged from a fuel control unit (FCU) and delivered for combustion. However, an engine heat management system requires different conditions during engine operation. For example, an engine without heat transfer to the aircraft fuel tank via motive flow, requires a relatively larger FOHE during take off conditions than during altitude cruise conditions. It is also known in the art that contamination may occur in an oil system of gas turbine engines and said contamination may block the relatively narrow passages within an FOHE system, thereby blocking the entire oil system. Therefore, a conventional FOHE system may be entirely bypassed by a bypass valve which opens only when the FOHE becomes blocked. When the bypass valve is open, the entire FOHE system does not function and the heat exchange between the hot oil and the cold fuel does not take place. Therefore, there is a need to provide an improved fuel oil heat exchange system for gas turbine engines.